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URBANA 


STATE  OF  ILLINOIS 
DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

A.  M.  SHELTON.  Director 

DIVISION  OF  THE 
STATE  GEOLOGICAL  SURVEY 

M.  M.  LEIGHTON.  Chief 
REPORT  OF  INVESTIGATIONS-NO.  12 


LIMESTONE  FOR  SEWAGE  FILTER  BEDS 

CAUSES  OF  DISINTEGRATION,  DESIRABLE  PROPERTIES, 
AND  METHODS  OF  TESTING 

BY 

J.  E.  LAMAR 


PRINTED  BY  AUTHORITY  OF  THE  STATE  OF  ILLINOIS 


URBANA,  ILLINOIS 
1927 


STATE  OF  ILLINOIS 
DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

A.  M.  SHELTON.  Director 

DIVISION  OF  THE 

STATE  GEOLOGICAL  SURVEY 

M.  M.  LEIGHTON.  Chwf 


Committee  of  the  Board  of  Natural  Resources 
and  Conservation 

A.  M.  Sheltox,  Chairman 

Director  of  Registration  and  Education 

Charles  M.  Thompson 

Representing    the    President   of    the    Uni 
versity  of  Illinois 

Edson  S.  Bastin 
Geologist 


Schnepp  &  Barnes,  Piunteiss 

Springfield,  III. 

I  !i  2  7 

65102     ■:: 


LIMESTONE  FOR  SEWAGE  FILTER  BEDS 

CAUSES  OF  DISINTEGRATION,  DESIRABLE  PROPERTIES, 
AND  METHODS  OF  TESTING 

By  J.  E.  Lamar 

OUTLINE 

PACiE 

Introduction 5 

General    statement 5 

Acknowledgments    5 

Causes  of  disintegration  of  filter  stone 6 

Mechanical   disintegration 6 

Freezing  and   thawing 6 

Heating   and   cooling 7 

Physical  factors  influencing  the  rate  of  mechanical  disintegration....  7 

Porosity    7 

Texture     7 

Bedded  or  laminated  structure 8 

Chemical    disintegration 8 

Solution    : 8 

Oxidation     9 

Hydration    10 

Cooperative  effect  of  mechanical  and  chemical  disintegration 10 

Properties  desirable  in  filter  stone 10 

Testing  of  limestone  and  dolomite  filter  stone 11 

Tests  for  hardness,  toughness,  and  wear 11 

Accelerated   soundness   test 13 

Freezing    test 15 

Water  absorption  test 15 

Microscopic   examination 15 

Solution    tests 15 

Etching    15 

Rate  of  solution 16 

Studies    of   residue 16 

Chemical    analysis 16 

ILLUSTRATIONS 

FIGURE  PAGE 

1-5.     The  effect  on  different  types  of  limestones  of  etching  for  the  same  time 
period  and  with  the  same  acid  concentration. 

1.  Niagaran    dolomite IS 

2.  Ste.    Genevieve    oolitic    limestone 19 

3.  Chester   limestone 20 

4.  Chester   limestone 21 

5.  Pennsylvanian  limestone 21 

TABLES 

PAGE 

1.  Variations  in  physical  tests  of  Niagaran  dolomite 12 

2.  Hypothetical  accelerated  soundness  tests 14 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  Illinois  Urbana-Champaign 


http://archive.org/details/limestoneforsewa55712lama 


INTRODUCTION 

General  Statement 

The  matter  of  predetermining  the  suitability  of  a  given  limestone  for 
use  in  sewage  filter  beds  is  one  which  is  yearly  coming  to  be  of  more  interest 
to  quarrymen  and  engineers  of  sanitation.  This  interest  is  linked  with  the 
growth  of  towns  and  cities,  the  consequent  increase  in  the  amount  of  sewage, 
and  the  endangering  of  public  health  by  improper  sewage  disposal.  As  a 
result,  the  use  of  sprinkling  and  trickling  filters  has  come  to  be  of  increas- 
ing interest.  In  a  filter  recently  constructed  for  a  town  of  25, 000  people, 
6S0  carloads  or  about  13  trains  of  limestone  were  used.  It  is  apparent, 
therefore,  that  the  initial  cost  of  such  a  quantity  of  stone  is  an  important 
item  to  a  municipality,  and  that  an  enduring  stone  is  desirable  not  only 
because  of  initial  cost  but  because  of  the  cost  of  removing  poor  stone  and 
replacing  it  with  new  material.  The  problem  of  sewage  disposal  during 
the  time  required  for  replacement  of  the  filter  stone  is  still  another  factor 
making  this  operation  one  to  be  assiduously  avoided. 

The  study  of  limestones  to  determine  their  suitability  as  filter  stone 
is  new  and  still  in  the  experimental  sta.^e.  As  yet,  very  few  data  are  avail- 
able which  correlate  laboratory  tests  and  the  actual  life  of  the  stone  in  the 
filter  bed.  The  collection  of  such  data  should  be  the  ultimate  aim  of  research 
and  investigation  in  this  line.  However,  the  processes  operating  to  destroy 
the  limestone  and  their  action  on  the  stone  may  be  evaluated,  and  such  tests 
as  are  at  hand  for  determining  the  resistance  of  the  stone  to  destructive 
agencies  may  be  applied.  Although  the  only  real  test  of  a  filter  stone  is  its 
behavior  in  the  filter  bed,  the  results  of  the  laboratory  tests  give,  at  the 
present  time,  the  most  intelligent  basis  for  judging  the  comparative  merits  of 
proposed  filter  stones. 

The  function  of  filter  stone  in  a  sewage  filter  is  essentially  that  of  a 
lodging  place  for  bacteria  which  gather  and  grow  upon  the  surface  of  the 
stone,  and  by  their  life  processes  effect  a  purification  of  the  sewage. 

Acknowledgments 

The  author  wishes  to  acknowledge  with  thanks  the  helpful  criticisms 
and  suggestions  bearing  on  this  paper,  made  by  Dr.  A.  M.  Ruswell,  Chief, 
Illinois  State  Water  Survey,  and  Dr.  Ah  M.  Leighton,  Chief,  Illinois  State 
Geological  Survey. 


b  LIMESTONE   FOB   SEWAGE  FILTEB    BEDS 

CAUSES  OF  DISINTEGRATION  OF  FILTER   STONE 

The  principal  processes  effecting  the  destruction  of  filter  stone  in 
sprinkling  and  trickling  filters  are  mechanical  disintegration  and  chemical 
disintegration.  These  two  processes  though  they  are  doubtless  greatly 
cooperative  in  their  action,  will  be  discussed  separately,  and  their  combined 
activity  will  be  taken  up  in  a  subsequent  statement.  Although  their  destruc- 
tive effect  is  slow, — not  to  be  measured  in  single  years — yet  ultimately  and 
in  the  aggregate  these  processes  do  produce  a  greater  or  lesser  destruction 
of  limestone  filter  media. 

Mechanical  Disintegration 

freezing  and  thawing 

The  most  important  process  producing  mechanical  disintegration  i- 
freezing  and  thawing.  The  destructive  effect  of  this  process  depends  on 
the  number  of  times  it  is  repeated  which  in  turn  is  related  to  the  climate  and 
to  the  operation  of  the  filter  bed.  During  the  winter  months  filters  may  be 
operated  continuously,  or  only  during  the  daytime,  to  treat  the  more  con- 
centrated sewage,  or,  if  the  dilution  is  sufficient,  the  sewage  may  be  In- 
passed  entirely.  In  continuous  operation  in  a  moderate  climate  a  certain 
amount  of  ice  forms  usually  in  wide  circles  around  the  nozzles  of  the 
sprinkler  and  at  the  surface  of  the  filter  bed.  The  temperature  of  the 
sewage  and  the  biological  action  are  usually  sufficient  to  prevent  frost  from 
entering  to  any  considerable  depth. 

Under  severe  conditions,  such  as  a  sudden  cold  wave  accompanied  by 
wind,  the  entire  bed  may  become  covered  with  ice.  Under  these  condition^  a 
given  portion  of  the  bed  may  freeze  and  thaw  four  or  five  times  during  the 
winter. 

In  filters  in  which  only  the  strong  day  sewage  is  treated  and  the  nighl 
sewage  is  by-passed,  the  exposed  portion  of  the  bed  may  freeze  at  night  and 
thaw  again  when  treated  with  the  warm  sewage  so  that  freezing  and  thaw- 
ing under  certain  weather  conditions  might  occur  daily,  thus  affecting  tin 
filter  stone  more  severely. 

Filters  which  are  not  operated  during  the  winter  months,  should  ft 
shut  down  in  time  to  avoid  the  earliest  probable  freezing  temperature,  so 
that  they  may  drain  completely  and  dry  out. 

Freezing  and  thawing  disrupts  a  stone  principally  by  reason  of  the  fad 
that  when  water  changes  to  ice  it  expands  about  1/10  its  volume.  There- 
fore, if  a  stone  is  saturated  with  water,  and  the  water  freezes,  the  stone 
will  be  disrupted  unless  it  has  sufficient  strength  to  overcome  the  force  of 
the  expansion  accompanying  the  change  of  water  to  ice  in  the  sub-surface 
pores.     In  the  surface  pores  the  expansion  accompanying  freezing  is  prob- 


LIMESTONE  FOR    SEWAGE   FILTER    BEDS  7 

iblv  partly  accommodated  in  the  open  direction  of  the  pore,  but  not  always 
sufficiently  to  eliminate  disruptive  stresses  completely.  Though  a  stone  may 
withstand  the  forces  occasioned  during  the  freezing  of  water  in  its  pores 
nid  in  pits  or  pockets  on  its  surface  for  a  number  of  repetitions  of  the  pro- 
:ess,  it  eventually  gives  way  and  fractures  or  chips.  Inasmuch  as  there  is 
inherently  less  resistance  to  breakage  in  the  surface  portions  of  a  stone,  other 
things  being  equal,  the  common  effect  of  freezing  and  thawing  is  a  chipping 
or  scaling  off  of  the  surface. 

HEATING  AND  COOLING 

Although  not  nearly  so  important  as  freezing  and  thawing,  the  expan- 
sion and  contraction  of  the  stone  in  a  filter  bed,  as  the  result  of  heating  by 
the  sun's  rays  in  the  summer  and  subsequent  cooling  by  dousing  with  sew- 
age, is  doubtless  a  contributory  factor  in  the  destruction  of  filter  stone.  The 
: effect  on  the  stone  would  probably  be  to  cause  it  to  chip  or  spall. 

PHYSICAL  FACTORS  INFLUENCING  THE  RATE  OF  MECHANICAL  DISINTEGRATION 

POROSITY 

The  pores  of  a  stone  which  can  be  penetrated  by  a  liquid  must  have  a 
connection  with  the  surface  of  the  rock  and  therefore  the  amount  of  water 
a  stone  can  absorb  is  related  to  the  volume  of  the  connected  pore  space  it 
possesses.  This  porosity  is  an  important  factor  governing  the  effectiveness 
of  freezing  and  thawing  in  destroying  the  stone.  The  distribution  and  size 
of  the  connected  pores  is  also  important  in  this  regard.  For  example  two 
stones  may  have  the  same  porosity,  the  first  with  evenly  distributed  small 
pores,  and  the  second  with  but  a  few  large  pores  concentrated  in  a  given 
zone ;  the  second  would  fracture  more  quickly  than  the  first,  other  things 
being  equal,  because  of  the  concentration  and  localization  of  the  stresses  set 
up  in  the  few  larger  pores  of  the  stone  by  repeated  freezing  and  thawing. 

TEXTURE 

Limestones  in  general  may  be  grouped  into  three  main  classes  accord- 
ing to  their  texture  as  apparent  to  the  naked  eye,  namely,  crystalline,  non- 
crystalline or  dense,  and  granular.  There  are  some  limestone  formations 
which  fall  definitely  into  one  of  these  three  classes,  but  in  general  limestones 
have  such  a  variety  of  texture  that  any  two  or  even  all  of  the  above  classes 
may  be  embodied  in  one  formation.  The  terms  are  of  value,  however,  in 
expressing  the  relative  importance  of  the  three  textural  divisions  for  pur- 
poses of  discussion.  There  is  a  definite  relation  between  the  physical  struc- 
ture of  filter  stone  and  its  resistance  to  disruption,  for  the  strength  of  a 
crystalline  limestone  depends  on  the  degree  of  interlocking  of  the  crystals  or, 
if  the  stone  is  granular,  on  the  completeness  of  the  bonding  of  the  granular 


8  LIMESTONE   FOB   SEWAGE  I'll.TEH   BEDS 

material.  A  stone  which  appears  crystalline  to  the  eye  is  likely  to  have  well- 
interlocked  grains.  A  stone  which  appears  non-crystalline  may  or  may  not 
have  well-interlocked  crystals.  All  limestones  when  examined  microscopic- 
ally under  sufficiently  high  magnification  are  seen  to  he  composed,  to 
greater  or  lesser  extent,  of  crystals  of  calcite ;  if  the  stone  is  a  dolomite,  the 
crystals  are  calcite  and  dolomite.  Therefore,  some  limestones,  non-crystal- 
line to  the  eye,  are  simply  so  line  grained  that  the  individual  crystals  are  nol 
megascopically  recognizable.  In  many  limestones  appearing  to  the 
either  crystalline  or  non-crystalline,  though  more  frequently  the  latter.  cla\ 
is  present  in  varying  amounts.  This  clay,  in  general,  lessens  the  strength 
of  the  stone  because  it  interrupts  the  interlocking  of  grains  or  coats  them  at 
the  planes  of  contact.  In  some  rocks  the  clay  is  segregated  in  zones  or  thin 
hands  through  the  stone  which  often  constitute  planes  of  weakness.  In  the 
case  of  siliceous  limestones,  crystals  of  silica,  either  localized  or  evenly  dis- 
seminated through  the  stone,  will  be  found  interlocking  with  or  replacing 
calcite  crystals. 

In  granular  limestones  composed  of  fossils,  fossil  debris,  oolite  grains, 
or  calcareous  debris  of  any  sort,  the  strength  of  the  stone  depends  to  a  largi 
degree  on  the  completeness  of  the  bond  effected  between  the  granular  ma- 
terial and  the  matrix  and  on  the  strength  of  the  bonding  substance.  In  main 
limestones  the  bond  is  very  good;  in  others  a  thin  film  of  organic  material 
surrounds  many  of  the  granules  or  fossils  and  consequently  interrupts  the 
continuity  of  the  bond. 

BEDDED   OK    LAMINATED    STBUCTUBE 

Some  limestones  possess  a  distinct  bedded  or  laminated  structure  which 
exerts  an  important  influence  on  the  ability  of  the  stone  to  resist  disintegra 
(ion.  The  bedded  structure  is  not  the  gross  feature  of  bedding  hut  rather 
minor,  abrupt  variations  in  the  texture  of  the  granular  or  detrital  materials 
composing  the  stone  within  a  larger  bed.  The  laminated  structure  is  not 
generally  visible  to  the  eye  in  fresh  specimens,  hut  is  usually  revealed  during 
the  accelerated  soundness  test  by  a  flaking  or  splitting  of  the  stone  into  thin 
sheets  or  laminae.  Inasmuch  as  the  contacts  of  texturally  unlike  sedimentarj 
materials  are  often  planes  of  weak,  partly  or  wholly  interrupted  hond,  a 
stone  having  a  bedded  or  laminated  structure  should  he  thoroughly  tested 
before  it  is  accepted  as  filter  stone. 

Chemical  1  )isintegration 

solution 
As  long  as  a  sprinkling  filter  is  in  operation,  all  of  the  stone  in  the  filter 
bed  is  being  repeatedly  doused  with  the  effluent   from  the  settling  tanks.     In 
ordinary  domestic  sewage  this  effluent  is  largely   water,  hut  it  contains  car- 


LIMESTONE  FOR   SEWAGE  KILTKK   BEDS  9 

bonic  acid  and  possibly  a  number  of  fatty  acids.  Although  both  are  in  very 
dilute  solution  they  doubtless  dissolve  a  relatively  small  but  still  appreciable 
portion  of  the  filter  limestone  in  a  period  of  years.  Furthermore,  the  bac- 
teria growing  on  the  stone  produce  carbon  dioxide  which  forms  carbonic 
acid.  There  is  some  question,  however,  as  to  whether  the  net  effect  of  the 
coating  of  bacterial  jelly  is  not  more  protective  than  injurious  as  regards 
solution  of  the  filter  stone. 

Considering  the  effect  of  solution  on  limestones  and  dolomites,  it  is 
noteworthy  that  dolomites,  though  more  soluble  than  limestones,  are  less 
rapidly  soluble.  The  rate  of  solution  is  of  importance  because  the  move- 
ment of  the  water  through  the  filter  bed  defeats  any  chance  of  saturation  of 
the  dissolving  medium,  thus  nullifying  the  effect  of  differences  in  solubility, 
and  facilitates  the  solution  of  the  more  rapidly  soluble  substance.  The  rate 
nf  solution  of  a  stone,  however,  is  influenced  by  an  additional  factor,  namely, 
the  area  of  surface  exposed  to  the  solvent.  The  larger  the  area  the  greater 
the  potential  rate  of  solution.  In  general,  dolomite,  as  a  result  of  the  manner 
of  its  formation,  is  more  porous  than  limestone,  and  consequently  exposes 
a  greater  surface  to  the  solvent  than  limestone.  This  tends  to  and  may  prac- 
tically equalize  in  the  net  result  the  difference  between  the  two  rocks  in  rates 
of  solution. 

If  a  piece  of  limestone  or  dolomite  with  a  plane  surface  is  immersed  in 
hydrochloric  acid  for  a  brief  time,  it  will  be  found  that  certain  parts  of  the 
stone  have  dissolved  more  rapidly  than  others  (figs.  1-5,  pp.  18-21  ).  In  some 
stones,  the  fossils,  fossil  debris,  vein  fillings,  and  the  like  dissolve  the  more 
rapidly  and  in  others,  the  matrix.  In  still  others  the  effect  of  the  solution  is 
about  equal  over  the  area  exposed.  If  the  stone  is  very  pure,  solution  will 
leave  a  surface  nearly  similar  to  the  original  one  ;  if ,  however,  the  stone  is  an 
argillaceous  or  siliceous  limestone  or  dolomite,  a  protective  residual  coat  will 
remain.  If  the  residual  coating  is  firm  and  hard,  it  is  an  asset  in  hindering 
further  solution.  The  effect  of  selective  solution  on  any  given  portion  of 
the  stone  is  to  increase  the  surface  porosity.  As  a  result  additional  surface 
is  made  available  to  the  action  of  the  solvent,  and  the  opportunities  for  me- 
chanical disintegration  are  increased.  The  matter  of  selective  solution  is 
thought  to  be  the  crux  of  the  potential  destructive  effects  of  solution  on  filter 
stone.  A  stone  of  even  texture  and  uniform  solubility  would  apparentlv  be 
desirable  to  offset  the  effects  of  solution. 

OXIDATION 

Because  of  the  frequent  wettings  and  complete  or  partial  dryings  to 
which  filter  stone  is  subjected,  the  process  of  oxidation  is  greatly  facilitated. 
Substances  which  oxidize  are,  therefore,  to  be  consistently  avoided.  Prob- 
ably the  worst  offenders   in   this  class   are  pyrite  and  marcasite,  both   sul- 


10  LIMESTONE   FOB   SEWAGE   FILTEB   BEDS 

phides  of  iron.  These  oxidize  to  linn  mite,  the  hydrated  oxide  of  iron. 
which  occupies  more  volume  than  either  of  the  sulphides.  As  a  consequence 
oxidation  of  either  of  the  sulphides  to  limonite  is  accompanied  by  forces 
tending  to  disrupt  or  weaken  a  stone  if  they  cannot  he  accommodated  by  free 
outward  expansion  as  in  surface  pores. 

HYDRATION 

It  is  possible  that  some  limestones  contain  clay  minerals  along  bedding 
planes,  or  less  probably  in  a  disseminated  state,  which  absorb  water  as  a 
result  of  the  constant  wetting  to  which  they  are  subjected,  with  an  accom- 
panying increase  in  volume.  This  increase  would  have  the  same  general 
effect  as  that  resulting  from  the  change  of  pyrite  into  limonite. 

Cooperative  Effect  of  Mechanical  and  Chemical  Disixtegratiox 

As  previously  stated,  it  is  very  difficult  to  divorce  and  identify  singly 
the  effects  of  mechanical  and  chemical  disintegration,  respectively,  on  a  filter 
stone.  Mechanical  disintegration  increases  the  surface  area  available  for 
the  agents  of  chemical  disintegration.  Chemical  disintegration,  in  turn,  if 
it  is  selective,  results  in  an  increase  of  porosity  and  favors  further  mechan- 
ical disintegration.  Without  doubt  the  upper  foot  of  the  filter  bed  is  the 
zone  of  maximum  effect  of  combined  mechanical  and  chemical  disintegra- 
tion, ft  seems  likely,  also,  that  the  basal  two  or  three  feet  of  a  filter  bed  are 
the  most  favorable  for  chemical  disintegration  not  excepting  the  process  of 
oxidation. 

PROPERTIES  DESIRABLE  IN  FILTER  STOXE 

From  the  foregoing  discussion  of  the  agencies  causing  the  destruction 
of  filter  stone,  the  following  properties  seem  desirable  in  filter  stone : 

(1)  The  stone  should  have  a  minimum  volume  of  pore  space  connected  with 
the  surf-ace. 

(2)  The  pores  of  the  stone  should  be  small  and  evenly  distributed. 

(3)  The  stone  should  consist  of  well-interlocked  crystals  or  if  it  is  granular, 
the  grains  should  be  firmly  bonded  by  a  strong  cement. 

(4)  The  stone  should  be  of  uniform  solubility. 

(5)  The  stone  should  be  free  from  minerals  which  oxidize  or  hydrate.  Pyrite 
and  marcasite  especially  are  to  be  avoided. 

(6)  The  stone  should  have  a  sufficiently  rough  surface  to  furnish  anchorage 
for  the  bacteria  which  are  to  grow  upon  it. 

(7)  The  stone  should  be  comparatively  pure  chemically.  Stones  with  high 
clay  contents  are  generally  to  be  avoided.  A  high  siliceous  content  is  probably  not 
harmful  if  the  silica  occurs  in  fine  crystals  evenly  disseminated. 

(8)  The  stone  as  delivered  to  the  filtering  plant  should  be  free  from  dirt  or 
fine  rock  particles  which  might  collect  in  and  clog  the  basal  portion  of  the  filter  bed. 


LIMESTONE  FOE   SEWAGE   FILTER    liEDS  11 

TESTING  OF  LIMESTONE  AND  DOLOMITE  FILTER  STONE 

Certain  tests  may  be  made  on  limestones  and  dolomites  which  individu- 
ally indicate  the  relative  value  of  specific  properties  of  a  stone.  It  is  impos- 
sible, however,  exactly  to  duplicate  conditions  in  nature  and  at  the  same 
time  accelerate  in  a  laboratory  the  combined  effects  of  mechanical  and  chem- 
ical disintegration.  Therefore,  the  results  of  any  set  of  tests  are  indicative 
rather  than  absolute.  Nevertheless,  as  such  they  serve  a  purpose.  The 
following  series  of  tests,  though  doubtless  not  in  their  ultimate  form,  appear 
to  give  valuable  data  concerning  filter  stones  and  will  serve,  as  well,  as  a 
basis  for  devising  improved  tests. 

Tests  for  Hardness,  Toughness  and  Wear 

It  is  natural  in  developing  a  new  set  of  tests  for  a  certain  material,  to 
turn  to  known  tests  and  attempt  to  adapt  them  to  the  work  at  hand.  In 
an  endeavor  to  find  adequate  tests  for  filter  stone,  engineers  have  turned  to 
the  tests  made  on  limestone  highway  aggregates,  and  accordingly  have  em- 
ployed the  toughness  test,  the  Dorry  hardness  test,  the  Deval  abrasion  test 
and  the  accelerated  soundness  test.  Considering  the  first  three  tests  as  a 
whole,  they  indicate  the  resistance  of  rock  to  repeated  impact  and  to  wear 
and  abrasion.  Inasmuch  as  it  does  not  seem  probable  that  impact,  wear,  or 
abrasion  plays  any  important  part  in  the  destruction  of  filter  stone,  it  would 
appear  that  these  tests  might  be  dispensed  with.  However,  since  the  results 
of  the  hardness,  toughness  and  wear  tests  and  the  percentage  of  water 
absorbed  are  in  most  cases  the  only  data  available  from  the  numerous  tests 
of  various  stones  for  aggregate,  these  data  may  give  clues  as  to  range  and 
character  of  the  regional  variations  in  a  given  limestone  formation.  For 
example,  Table  1  gives  data  on  the  variation  in  the  physical  properties  of  the 
Xiagaran  dolomite  in  the  three  principal  areas  in  which  it  is  quarried  in 
Illinois.  From  the  table  it  appears  that  the  stone  from  district  No.  1  is 
likely  to  be  more  variable  than  the  stone  from  districts  Nos.  2  and  3  and 
consequently  stone  from  that  district  should  be  more  carefully  and  fre- 
quently sampled  than  stone  from  the  other  two  districts. 

Regarding  the  physical  tests  shown  in  Table  1,  only  the  water  absorp- 
tion, discussed  later,  is  individually  significant.  Although  it  may  be  said 
that  rocks  satisfactorily  passing  the  above  mentioned  tests  from  the  stand- 
point of  highway  material  will  probably  pass  the  accelerated  soundness  test. 
there  are  known  to  be  exceptions  which  make  it  highly  desirable  that  the 
soundness  test  itself  be  applied.  This  test,  if  applied,  practically  eliminates 
the  need  of  hardness,  toughness  and  wear  tests  for  filter  stone. 


12 


LIMESTONE   FOB   SEWAGE   FILTER    BEDS 


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LIMESTONE   FOR   SEWAGE   FILTiK    BEDS  13 


Accelerated  Soundness  Test 


The  accelerated  soundness  test,  also  known  as  Brard's  test  or  as  the 
quick  weathering  or  sodium  sulphate  test,  consists  of  immersing  a  repre- 
sentative sample  consisting  of  pieces  of  two-inch  stone  in  a  saturated  solu- 
tion of  sodium  sulphate  for  20  hours  and  then  drying  it  at  a  temperature 
of  100°  C.  for  4  hours.  The  process  is  repeated  5  times  in  testing  highway 
aggregate,  and  the  stone  showing  no  disintegration  is  considered  satisfactory 
Iggregate.  Stone  which  fails  under  this  test  is  considered  as  doubtful  until 
other  tests  prove  or  disprove  its  value. 

The  theory  of  this  test  is  that  the  saturated  sodium  sulphate  solution 
penetrates  the  pores  of  the  stone  during  immersion.  When  the  stone  is 
dried,  water  is  driven  off  and  the  sodium  sulphate  crystallizes  and  the  grow- 
ing crystals  set  up  stresses  within  the  stone.  The  process  simulates  in  net 
effect,  therefore,  the  action  of  the  freezing  of  water  in  the  pores  of  a  stone. 
The  crystallization  of  the  sodium  sulphate  in  the  pores  of  the  stone,  how- 
ever, appears  to  exert  a  greater  destructive  force  than  does  the  crystalliza- 
tion of  water,  and  as  a  consequence  the  test  is  much  more  severe  than  normal 
freezing.  Experiment  has  shown  that  sodium  sulphate  crystallizes  in  three 
different  forms  and  consequently  may  not  always  give  comparable  results. 
Sodium  chloride  is  therefore  suggested  as  a  substitute  of  similar  action  as  it 
crystallizes  in  one  form  only.1 

This  test  should  be  of  great  value  in  testing  filter  stone,  inasmuch  as  it 
simulates  natural  mechanical  disintegration  and  depends  for  its  effects  on 
essentially  the  same  factors.  The  amount,  distribution,  and  character  of 
the  porosity  are  involved,  as  are  also  matters  of  bond  strength,  and  crystal 
interlocking.  Since  filter  stone  is  subjected  to  more  severe  conditions  of 
mechanical  disintegration  than  is  ordinary  concrete  aggregate,  it  should  cer- 
tainly be  required  to  pass  a  minimum  of  five  repetitions  of  the  soundness 
test.    The  stone  not  passing  this  test  should  be  regarded  with  suspicion. 

The  U.  S.  Bureau  of  Standards  which  has  been  making  studies  of 
weathering  tests,  states  that  the  effect  of  one  crystallization  with  sodium 
chloride  is  equivalent  to  about  eight  water  freezings.2  Sodium  sulphate  is 
at  least  equally  severe  in  its  action.  There  is,  however,  considerable  varia- 
tion of  the  ratio  in  different  types  of  stone.  The  studies  of  the  Bureau  of 
Standards  have  been  confined  largely  to  limestones  and  sandstones  and,  to 
date,  indicate  a  general  average  of  about  1,000  freezings  to  produce  disinte- 
gration, though  some  specimens  have  shown  no  disintegration  after  2,000 
freezings.  These  results  suggest  that  with  a  correlative  value  of  1  to  8  for 
sodium  chloride  and  water  crystallizations,  the  ultimate  strength  of   many 


1  Kessler,  D.  W.,  Stone,  vol.  4fi.  No.  6,  pp.  351-353,  June,  1925. 

2  Idem. 


14 


.IMESTONE   l-'OU   SEWAGE    II  I.I  I  H    BEDS 


limestones  would  not  be  reached  with  less  than  Kill  repetitions  of  the  sound- 
ness test.  Possibly  the  best  method  of  application  of  this  test  is  to  continue 
treatment  of  a  representative  sample  of  50  or  100  pieces  of  stone  till  all  show- 
failure.  As  the  respective  fragments  fail,  they  could  be  removed  and  the 
number  of  immersions  and  dryings  recorded.  When  all  pieces  had  failed, 
an  average  could  be  struck  by  multiplying  the  number  of  pieces  failing  by 
the  number  of  immersions  and  dryings  withstood  without  failure  by  each, 
totalling  the  product  and  dividing  by  the  number  of  pieces  used.  This 
would  give  the  number  of  immersions  and  dryings  necessary  to  disrupt  an 
average  fragment  of  the  entire  sample.  The  stone  having  the  highest  aver- 
age number  would  theoretically  be  the  best.  For  example,  hypothetically. 
of  two  samples  failing  as  shown  in  Table  'i,  sample  No.  2  would  be  the  bet- 
ter stone. 

Table  2. — Hypothetical  accelerated  soundness  tests 


Sample  No. 

L 

Sample  No 

2 

No.  immer- 
sions and 
dryings 

No.  pieces 
failing 

Product 

No.  immer- 
sions and 
dryings 

No.  pieces 
failing 

Product 

11 

4 

44 

10 

6 

60 

15 

7 

105 

12 

6 

72 

20 

12 

240 

15 

8 

120 

21 

19 

399 

20 

10 

200 

22 

20 

440 

26 

12 

312 

24 

17 

408 

27 

12 

324 

28 

11 

308 

28 

28 

784 

31 

8 

248 

31 

16 

496 

48 

2 

96 

42 

1 

42 

44 

1 

44 

100 

100)2,288 

100 

100)2,454 

Av.  22.88 

Av.  24.54 

It  is  highly  desirable  that,  in  conjunction  with  this  accelerated  sound- 
ness test,  the  resistance  of  the  stone  to  weathering  in  the  natural  outcrop  be 
also  considered.  It  is  generally  possible  to  bud  outcrops  of  a  given  stone 
near  a  given  quarry  which  have  been  exposed  to  the  weather  for  many  years. 
From  these  outcrops  many  points  of  interest  may  be  gathered  concerning 
how  the  stone  is  likely  to  weather.  The  loose  material  at  the  base  of  a  cliff 
is  likely  to  yield  the  most  valuable  data. 


LIMESTONE  FOIt  SEWAGE  FILTER   BEDS  15 

Freezing  Test 
The  freezing  test,  consisting  of  alternately  freezing  and  thawing  water- 
saturated  specimens,  is  preferred  by  some  for  determining  the  weather  re- 
sistance of  a  stone.  It  is  a  slower  and  more  tedious  process  than  the  accel- 
erated soundness  test,  and  requires  much  more  apparatus.  It  is  somewhat 
doubtful  if  the  greater  accuracy  claimed  for  this  test  compared  with  the 
accelerated  soundness  test  warrants  the  extra  time  and  equipment  required 
in  testing  material  like  filter  stone  which  is  to  be  subjected  to  extremes  of 
mechanical  disintegration. 

Waieb  Absorption  Test 
This  test  is  designed  to  measure  the  volume  of  connected  pore  space  by 
determining  the  amount  of  water  absorbed  by  a  stone  in  24  hours.  A  repre- 
sentative sample  of  about  1000  grams  is  dried  at  100°  C.  to  110°  C.  to  a  con- 
stant weight,  and  its  weight  carefully  determined.  It  is  then  immersed  in 
water  for  24  hours,  removed,  surface  dried,  and  reweighed.  The  amount  of 
water  absorbed  divided  by  the  weight  before  absorption  multiplied  by  100 
gives  the  percentage  of  absorption.  This  test  is  of  great  value  as  supplemen- 
tary to  the  soundness  test. 

Microscopic  Examination 
It  is  often  possible  by  microscopically  studying  thin  sections  of  the 
typical  phases  of  a  limestone,  to  predict  the  general  results  of  the  soundness 
test  and  possibly  eliminate  a  large  amount  of  labor.  Such  examination 
reveals  the  size  of  the  crystals  and  the  condition  of  crystal  interlock  or,  if 
the  stone  is  granular,  the  character  of  the  granules  and  the  bond.  Minute 
clay  partings,  grains  of  pyrite  or  other  foreign  minerals,  and  the  distribution 
and  size  of  the  pores  are  also  revealed.  Hirschwald  has  proposed  a  classi- 
fication1 of  limestones  and  dolomites,  repeated  by  Howell4  which  divides 
these  rocks  into  24  different  groups,  into  one  of  which  almost  every  lime- 
stone may  be  placed.  This  classification  may  be  used  to  standardize  terms 
descriptive  of  texture,  but  the  interpretation  of  the  phenomena  shown  in  a 
thin  section  depends  chiefly  on  the  skill  of  the  examiner  and  his  knowledge 
of  applied  sedimentary  petrology. 

Solution  Tests 

etching 

Under   the   discussion    of    chemical    disintegration,   the    various    effects 

produced  by  selective  solution  have  been  pointed  out.     In  order  to  determine 

bow  a  limestone  or  dolomite  will  dissolve,  a  number  of  representative  pieces 


'Hirschwald,  J.,  Handbuch  der  Bautechnischen  (lesteinspriif  ting,  Berlin,  1912. 
4  Howell,    J.    V.,    Notes    on    the    pre-Permian    Paleozoics    of    the    Wichita    Mountain 
area:  Am.  Assoc.  Pet.  Geol.   Bull.,  vol.   6,  No.   5,  pp.   412-425,  1922. 


16  LIMESTONE    FOR   SEWAGE   I'lI.TKH    BEDS 

of  stone  should  he  ground  so  that  each  piece  will  have  one  Hat  surface.  The 
pieces  should  he  selected  so  that  the  flat  surfaces  constitute  sections  parallel 
to  and  at  right  angles  to  the  bedding.  These  pieces  should  be  set  with  the 
flat  surfaces  up  and  parallel  to  the  bottom  of  a  shallow  dish,  to  prevent  the 
production  of  irregularities  by  carbon  dioxide  currents,  and  then  covered 
with  a  10  per  cent  solution  of  hydrochloric  acid.  The  etching  action  of  the 
acid  should  be  allowed  to  proceed  until  its  effect  is  clearly  apparent  on  the 
smoothed  surface.  The  distribution  and  character  of  argillaceous  and 
siliceous  materials,  partings,  sand  grains,  and  pyrite  are  usually  revealed  after 
etching  (figs.  L-5,  pp.  L8-21).  The  stone  showing  the  minimum  of  unde- 
sirable impurities  and  the  minimum  development  of  porosity,  or.  otherwise 
stated,  the  most  even  solution,  is  likely  to  be  the  more  desirable,  other  things 
being  equal. 

RATE  OF  SOLUTION 

The  only  truly  satisfactory  method  for  determining  the  rate  of  solution 
of  filter  stones  is  to  subject  a  representative  and  carefully  weighed  sample 
to  continual  dousing  with  sewage  water  for  a  period  of  weeks  and  then  to 
clean  it  thoroughly  and  weigh  it.  This  process  often  takes  too  long  to  be 
of  service  where  an  immediate  report  is  necessary.  Comparisons  may  he 
made  of  limestones  as  a  group,  and  similarly  by  dolomites,  by  cutting  cubes 
of  stone  of  the  same  size,  weighing  them,  and  immersing  them  in  very  dilute 
hydrochloric  acid  for  a  given  time.  The  loss  of  weight  gives  a  basis  Eol 
calculating  the  amount  which  has  dissolved.  No  tests  have  been  developed 
for  determining  the  relative  rates  of  solution  of  limestone  and  dolomite  in 
cold  dilute  hydrochloric  acid,  in  which  limestone  is  more  rapidly  soluble 
than  dolomite. 

Studies  of  Residue 

About  100  grams  of  stone  as  representative  of  the  deposit  as  possible, 
crushed  to  pass  a  10-mesh  sieve  and  be  retained  on  a  20-mesh  sieve,  is  treated 
with  hvdrochloric  acid  until  effervescence  ceases.  The  residue  is  collected 
on  a  filter  paper,  washed  free  from  acid,  dried  and  weighed.  The  relative 
proportions  of  such  materials  as  clay,  silt  or  sand,  are  recorded  as  well  as 
other  items  of  interest  concerning  the  presence  of  secondary  silica,  chert  or 
flint  and  mineral  grains.  The  presence  of  pyrite  or  marcasite  is  readily 
determined  in  this  test.  It  is  indicated  in  the  chemical  analysis  as  iron  and 
sulphur,  but  such  an  analysis  does  not  differentiate  between  iron  present  as 
the  hvdroxide  and  sulphur  present  in  compounds  other  than  iron  sulphide. 

Chemical  Analysis 

A  chemical  analysis  showing  the  percentages  of  the  following  com- 
pounds  in   limestones   and    dolomites   is    important   because   it   indicates   the 


LIMESTONE  FOB   SEWAGE   FILTER   BEDS  17 

general  composition  of  the  rock  for  filter  stone:  calcium  carbonate,  mag- 
nesium carbonate,  iron  oxide,  alumina,  silica,  and  sulphur.  These  com- 
pounds are  involved  in  a  consideration  of  the  resistance  of  a  stone  to  disin- 
tegration. It  is  recommended  that  these  tests  be  made  in  accordance  with 
the  procedure  outlined  by  Hillebrand  for  the  analysis  of  carbonate  rocks.5 


'  Hillebrand,  W.  P.,  The  analysis  of  silicate  and  carbonate  rocks:   U.   S.  Geol.   Survey 
Bull.   700,   1019. 


LIMESTONE   F<>|{   SEWAGE    FILTER    UEDS 


Figures  1-5  show  the  efifect  on  different  types  of  limestones  of  etching 
tor  the  same  time  period  and  with  the  same  acid  concentration. 


Fig.  1.  Niagaran  dolomite.  Maximum  surface  relief  about  1  mm.  The  etching 
has  enlarged  the  surface  pores  and  has  left  the  specimen  coated  with  fine  dolo- 
mite crystals  which  may  be  easily  brushed  off.  The  effect  of  surface  etching  or 
solution  of  the  calcium  carbonate  from  dolomitic  limestone  is  augmented  by  the 
consequent  freeing  of  dolomite  grains;  hence  the  actual  weight  of  dolomite  dis- 
integrated may  in  some  cases  be  equal  to  or  greater  than  the  weight  of  lime- 
stone dissolved  by  simple  solution  under  identical  conditions.  This  specimen 
chipped  slightly  after  8  repetitions  of  the  soundness  test,  but  thereafter  remained 
intact  through  the  remainder  of  20  repetitions.     ( Magnification  six  times.) 


LIMESTONE   l'OU   SEWAGE  FII.TKK   I1EDS 


19 


Fig.  2.  Ste.  Genevieve  oolitic  limestone.  Surface  relief  about  0.2  mm.  The 
etching  has  affected  the  stone  very  evenly  but  was  slightly  more  pronounced  on  the 
white  outer  portions  of  the  oolite  grains.  Near  the  center  of  the  right  margin  of 
the  illustration,  a  hexagonal  quartz  grain  which  is  the  center  of  an  oolite  grain 
projects  above  the  general  level  of  the  surface  and  casts  a  pronounced  shadow. 
That  this  specimen  showed  no  disintegration  after  20  repetitions  of  the  soundness 
test  indicates  a  good  bonding  of  the  granular  material  of  the  limestone.  (Magnifi- 
cation six  times.) 


20 


LIMESTONE    FOK   SEWAGE   FILTER    UEDS 


Fig.  3.  Chester  limestone.  Maximum  surface  relief  about  1  mm.  This  is  a  very 
pure  limestone  composed  of  fossil  debris  bonded  by  calcium  carbonate.  Etching 
has  dissolved  the  calcium  carbonate  and  Left  the  fossil  debris  projecting  from  the 
general  level  of  the  surface.  At  the  right  of  the  picture  is  a  brachiopod  which 
has  been  filled  with  calcite.  After  16  repetitions  of  the  soundness  test  this  speci- 
men broke  into  three  pieses  along  a  large  fossil.  There  was  no  breakage  during 
the  four  subsequent  repetitions  of  the  test.  The  resistance  of  this  specimen  to 
the  soundness  test  is  evidence  that  in  general  it  is  well  bonded.  (Magnification 
six  times.) 


LIMESTONE   FOIS   SEWAGE    FILTKH    BEDS 


21 


Fm.  4.  Chester  limestone.  Maximum  surface  relief  about  1  mm.  Etching  has 
removed  the  calcium  carbonate  cement  and  left  the  clayey,  siliceous  impurities  of 
the  stone  as  projections  above  the  general  surface.  Disintegration  of  this  speci- 
men began  after  4  repetitions  of  the  soundness  test,  and  complete  failure  resulted 
'  after  8  repetitions.  The  disintegration  of  the  stone  into  a  coarse  sand  showed 
that  the  bonding  material  was  weak.     (Magnification  six  times.) 


%m 


i 


Pig.  5.  Pennsylvanian  limestone.  Maximum  surface  relief  about  l1^  mm.  This 
is  a  siliceous  limestone  containing  fossils  composed  of  calcium  carbonate.  These 
were  dissolved  during  the  etching  and  gave  rise  to  the  linear  depressions.  After 
6  repetitions  of  the  soundness  test  this  specimen  began  to  chip  and  split  along 
the  surfaces  of  fossils.  After  11  repetitions,  disintegration  of  the  entire  specimen 
was  complete.  This  sample  shows  the  type  of  results  which  may  be  expected  in 
limestone  with  a  localized  porosity.     (Magnification  six  times.) 


CHliR'S" 
LIBRARY  BINDERS 
507  S.  Goodwin 
Urbano,IlL 


